The present invention relates generally to an oil filled pressure transducer and a method for making such a transducer and more particularly, to an oil filled pressure transducer having a lower backpressure when temperature is increased.
Oil filled pressure transducers are well known. The prior art shows such devices that have been used with oil, which oil or fluid is employed as a force-transmitting medium. In such devices, the pressure is applied to a transducer diaphragm, and typically these devices employ a metal diaphragm as a force collector. Such oil filled transducers are known in the art. For example, see U.S. Pat. No. 4,406,993 entitled, xe2x80x9cOil Filled Pressure Transducersxe2x80x9d issued on Sep. 27, 1983 to Anthony D. Kurtz, the inventor herein, and assigned to Kulite Semiconductor Products, Inc., the assignee herein. See also U.S. Pat. No. 6,330,829 entitled, xe2x80x9cOil Filled Pressure Transducerxe2x80x9d issued Dec. 18, 2001 to A. D. Kurtz et al., the inventor herein, and assigned to Kulite Semiconductor Products, Inc., the assignee herein.
It is well known that the deflection (xcex4)of a clamped edge diaphragm of thickness (t) and radius (a)is given by:   δ  =            3      ⁢      P      ⁢              xe2x80x83            ⁢                        a          4                ⁡                  (                                    m              2                        -            1                    )                            16      ⁢              Em        2            ⁢              t        3            
Where m=reciprocal of Poisson""s Ration
P=pressure
E=Young Modulus
In an oil-filled pressure transducer, the sensor element is isolated from the pressure media by means of a thin metal diaphragm and the pressure is transmitted to the sensor by means of a volume of oil mainly in the form of a film between the metal diaphragm and the sensor. This is, of course, indicated in the above-noted patents. In any event, when the temperature increases, the oil expands and pushes against the metal diaphragm thus exerting a backpressure against the sensor leading to an error signal. This problem is known and is addressed and explained in U.S. Pat. No. 5,999,082 entitled, xe2x80x9cCompensated Oil Filled Pressure Transducerxe2x80x9d issued on Dec. 7, 1999 to A. D. Kurtz et al., the inventor herein and assigned to the assignee herein. In that patent, the object was to reduce errors at very low pressure caused by the oil exerting a tension on the deflecting portion of the diaphragm.
It is an object of the present invention to reduce the oil volume to a minimum volume expansion and thus reduce the error causing backpressure.
Referring to FIG. 1, there is shown a prior art oil filled pressure transducer. In FIG. 1 the sensor 10 is mounted on a header assembly or header shell 11. This is usually accomplished by mounting the sensor 10 with the contact areas exposed and ball-bonding gold wires form the contact areas to the pins of the header. In FIG. 1, the pins of the header are designated by the reference numeral 12 and the contact areas 14 of the sensor 10 are bonded by means of gold wires 15 to the terminal areas 17, which are connected to the output or header pins 12. The header shell 11 has a top opening surrounded by a peripheral flange 21. A metal diaphragm (not shown) is secured to cover the opening.
To reduce the oil volume, an annular ceramic substrate 18, contains apertures 20 for the pins 12 and terminal areas 17. The substrate 18 is mounted on the header and the sensor 10 is mounted within the central hole 13 of the ceramic substrate 18. The thickness of the ceramic is chosen to be slightly less than the height of the pins 12, but slightly thicker than the height of the sensor. This is shown in FIG. 3B.
Referring to FIG. 3A, there is shown a top view of the prior art sensor of FIG. 1 and a cross sectional view in FIG. 3B showing the ceramic header with the sensor contact area 14 bonded to the pin 12 by a gold wire 15. It is also seen in FIG. 1 and FIG. 3 that there is an oil tube 22 which enables one to place oil within the sensor. As seen in FIG. 3A, there is an oil cavity 25 about each pin, there is an oil cavity 26 about the ceramic substrate and there is an oil cavity 28 around the sensor. This is also shown clearly in the view of FIG. 1. One can also see in FIG. 3B that the thickness of the ceramic substrate 18 is slightly less than the height of the pins 12, but slightly thicker than the height of the sensor. As seen, a wire 15 bonds the contact area 14 of the sensor to the terminal area 17 of the pin 12.
However, when ball-bonds are made to the gold wire which connects the sensors to the pins, both the wire and the ball-bonds are slightly elevated (looped) with respect to the ceramic 18. To prevent the metal isolation diaphragm 40 (FIG. 3B) which covers the top of the sensor from touching the wires 15 or the ball-bonds, a gap on the order of 0.01 to 0.015 inches must be left between the ceramic substrate 18 and the metal isolation diaphragm 40. This criterion contributes to increase the oil volume. In addition, the ceramic substrate 18 is mounted in a recess in the header 11 in order to insert the ceramic 18 over the pins 12, there must be a small difference in the inner diameter of the recess and the outer diameter of the ceramic 18. This also contributes to an increase in oil volume. It is therefore desirable to decrease the oil volume to an absolute minimum to avoid or reduce backpressure.
An oil filled pressure transducer utilizes a leadless sensor which is secured to a header comprising a glass pre-form and a header shell. The glass pre-form contains holes which accept header pins and another aperture or hole which accepts the oil fill tube. The diameter of the sensor is chosen to be almost as large as the inner diameter of the shell. In this manner, there is a small cut out over the portion of the sensor that would otherwise cover the oil fill tube. The sensor is mounted to the header using glass bonds. There is a very small space between the outer diameter of the sensor and the inner diameter header housing, which is filled with glass used to mount the sensor. Since there are no ball bonds or gold wires in the area between the surface of the sensor and the diaphragm, the distance between the sensor and metal diaphragm is drastically reduced, thereby substantially reducing the backpressure problem.